In triangle $ABC$, $AB = 11$, $AC = 13$, and $BC = 20$.  The medians $AD$, $BE$, and $CF$ of triangle $ABC$ intersect at the centroid $G$.  Let $P$ be the foot of the altitude from $G$ to $BC$.  Find $GP$.

[asy]
unitsize(0.3 cm);

pair A, B, C, D, E, F, G, P;

A = (44/5,33/5);
B = (0,0);
C = (20,0);
D = (B + C)/2;
E = (C + A)/2;
F = (A + B)/2;
G = (A + B + C)/3;
P = (G + reflect(B,C)*(G))/2;

draw(A--B--C--cycle);
draw(A--D);
draw(B--E);
draw(C--F);
draw(G--P);

label("$A$", A, dir(90));
label("$B$", B, SW);
label("$C$", C, SE);
label("$D$", D, SE);
label("$E$", E, NE);
label("$F$", F, NW);
label("$G$", G, NE);
label("$P$", P, SSW);
[/asy]
Let $Q$ be the foot of the altitude from $A$ to $BC$.  Then triangles $AQD$ and $GPD$ are similar.  Furthermore, \[\frac{GP}{AQ} = \frac{GD}{AD} = \frac{1}{3},\]so to find $GP$, we can find $AQ$.

[asy]
unitsize(0.3 cm);

pair A, B, C, D, E, F, G, P, Q;

A = (44/5,33/5);
B = (0,0);
C = (20,0);
D = (B + C)/2;
E = (C + A)/2;
F = (A + B)/2;
G = (A + B + C)/3;
P = (G + reflect(B,C)*(G))/2;
Q = (A + reflect(B,C)*(A))/2;

draw(A--B--C--cycle);
draw(A--D);
draw(B--E);
draw(C--F);
draw(G--P);
draw(A--Q);

label("$A$", A, dir(90));
label("$B$", B, SW);
label("$C$", C, SE);
label("$D$", D, SE);
label("$E$", E, NE);
label("$F$", F, NW);
label("$G$", G, NE);
label("$P$", P, S);
label("$Q$", Q, SSW);
[/asy]

The semi-perimeter of the triangle is $(11 + 13 + 20)/2 = 22$, so by Heron's formula, the area of triangle $ABC$ is $$\sqrt{22(22 - 11)(22 - 13)(22 - 20)} = 66.$$Hence, the height of triangle $ABC$ with respect to base $BC$ is $AQ = 2 \cdot 66/BC = 2 \cdot 66/20 = 33/5$.  Therefore, $GP = AQ/3 = (33/5)/3 = \boxed{\frac{11}{5}}$.